Presently there are several well known methods for fabricating integrated circuits on semiconductor wafers. In general, all methods somehow alter the surface of the wafer (i.e. substrate) to create the pattern of the integrated circuit on the surface. Of the presently known methods, perhaps the most well known and widely used methods involve either optical lithography systems, electron beam exposure systems (EBES), or electron projection systems (EPS). All of these systems, however, have certain functional limitations which affect their utility.
Optical lithography systems rely on photographic projections of the integrated circuit pattern on the substrate surface. As optical systems which rely on light to define the circuit pattern, however, they are limited in their resolution by the wavelength of light. Effectively, if the feature size is smaller than approximately two tenths of a micron (&lt;.about.0.2.mu.m) optical imaging is no longer useful. Although EBES type systems can provide better resolution than optical lithography systems for small feature sizes, since they do not have the wavelength limitations of optical lithography, their speed of operation is relatively slow. This is due to the fact that in an EBES type system one electron source must sequentially address each pixel in the integrated circuit pattern. For this reason, EBES type systems have not generally been used for the fabrication of semiconductor wafers. Instead, EBES type systems have been limited to the fabrication of masks which can be subsequently used in other type systems to fabricate semiconductor wafers. Unlike either optical lithography or EBES systems, EPS type systems have both good feature resolution and speed of operation. Present EPS type systems, however, have a very short operational life (less than fifty exposures). Furthermore, reconstruction of an EPS system must generally be accomplished in situ, and it is typically quite a cumbersome task.
The present invention has realized that a field emission array (FEA), of a type generally known in the pertinent art, can be incorporated into a digital electron lithography (DEL) system and, thereby, overcome limitations of the various systems discussed above. This incorporation, however, presents certain difficulties which must also be overcome in order to provide a system which is capable of rapidly and repeatedly generating high resolution patterns in a sustained operation. Specifically, a focusing component is required for the system which will direct electrons onto a substrate with such precision that the spot size of the focused electrons is smaller than the size of the pixel to be produced. Additionally, because each cathode in the FEA is significantly larger than the size of individual pixels in the pattern to be produced, each cathode must affect more than one pixel and, therefore, each cathode must be individually scanned to affect a plurality of pixels in a pixel matrix. Further, the electron energy must be sufficiently high to appropriately alter the substrate surface. Still further, insofar as the FEA itself is concerned, there are two primary concerns which must be addressed. First, although conventional FEAs require ultra high vacuum operational environments in order to avoid unacceptable deterioration of the cathodes in the FEA, it is preferable if a DEL system is able to operate in conventional vacuums. Second, there needs to be some realistic way in which to compensate for defective cathodes in the FEA.
In light of the above it is an object of the present invention to provide a digital electron lithography system which is capable of rapidly and repeatedly generating high resolution patterns on the surface of a substrate in a sustained operation. Another object of the present invention is to provide a digital electron lithography system which focuses electrons to spot sizes on a substrate which are smaller is size than the pixel being produced. Still another object of the present invention is to provide a digital electron lithography system which steers electrons from each individual cathode in a field emission array toward a plurality of pixels in a pixel matrix. Yet another object of the present invention is to provide a digital electron lithography system which can use a field emission array in a conventional vacuum environment. Also, another object of the present invention is to provide a digital electron lithography system which has the capability of first determining where there are defective cathodes in a field emission array and, then, compensating for these defective cathodes during operation of the system.